Device for achieving permanent equilibration in tower cranes

ABSTRACT

A device for obtaining permanent equilibration in tower cranes of the type including a load jib, over which a carriage fitted with a lifting hook travels, and a jib over which a carriage carrying a counterweight is adapted to be shifted, said device including in combination: means for measuring the moment resulting from the weight of the load, on the one hand, and from the position of the load carriage on the jib, on the other hand, means for shifting the counterweight carrier on the corresponding jib, and means for increasing the duration of the loading operation in order to enable the counterweight carrier to move only slowly and progressively as the load moment varies, without any disorderly motions.

United States Patent Durand 1 Feb. 15, 1972 [54] DEVICE FOR ACHIEVINGPERMANENT EQUILIBRATION IN TOWER CRANES [21] Appl. No.: 39,241

FOREIGN PATENTS OR APPLICATIONS 1,135,807 5/1957 France ..212/491,212,461 3/1960 France ..212/48 Primary Examiner-Harvey C. HomsbyAssistant Examiner-Merle F. Maffei Att0rneyWaters, Roditi, Schwartz &Nissen [57] ABSTRACT A device for obtaining permanent equilibration intower cranes of the type including a load jib, over which a carriagefitted with a lifting hook travels, and a jib over which a carriagecarrying a counterweight is adapted to be shifted, said device includingin combination: means for measuring the moment resulting from the weightof the load, on the one hand, and from the position of the load carriageon the jib, on the other hand, means for shifting the counterweightcarrier on the corresponding jib, and means for increasing the durationof the loading operation in order to enable the counterweight carrier tomove only slowly and progressively as the load moment varies, withoutany disorderly motions.

2 Claims, 5 Drawing Figures PATENTEBFEB 15 I973 SHEEI 1 [IF 3 FIG/1DEVICE FOR ACHIEVING PERMANENT EQUILIBRATION IN TOWER CRANES The presentinvention relates to a device for obtaining permanent equilibration intower cranes.

For many years, attempts have been made for achieving this result byshifting the counterweights of such cranes.

Many methods and devices have been devised for the purpose, but all ofthem come up against the problem set by the load being loaded and liftedwithin a very short time, which operations correspond to extremepositions of the counterweight. Moreover, the balance position duringthe operation is disturbed by dynamic effects, the latter beingamplified by the overall elasticity of the crane.

The present invention relates to a device enabling a permanent balanceto be obtained, whatever the load hanging from the hook of the carriageand the position of said carriage on the jib may be, this result beingachieved by causing the counterweight carrier to move over thecorresponding jib.

Said device for balancing a tower crane permanently includes incombination: means for shifting the counterweight carrier along thecorresponding jib, means for measuring the moment resulting from theweight of the load, on the one hand, and from the position of the loadcarriage on the jib, on the other hand, and means for increasing theduration of the loading operation in order to enable the counterweightcarrier to move only slowly and progressively as the load moment varies,without any disorderly motions.

In one embodiment of the invention, the means for measuring the loadmoment are disposed on the tie connecting the load jib to the top of thetower, and in the most common cases they consist of extensometers orsome other stress-measuring devices.

Furthermore, the means for shifting the counterweight carrier consist ofa speed-reducing motor unit.

In a preferred embodiment of the invention, the increase in the durationof the loading operation is obtained by increasing the length of thefixed end of the hoist cable by means of various windings, on the onehand, and by using an arrangement temporarily slackening the tension ofsaid cable, on the other hand.

This increase in the duration of the loading operation is veryimportant, because it gives the counterweight the time to move toachieve the equilibration before the load is actually lifted or laiddown.

Preferably, the arrangement for slackening the tension of the cabletemporarily is hydraulic, and comprises a multistrand block interposedbetween the fixed point of the hoist cable and the lifting hook, whereina first set of sheaves is fast, while a second set of sheaves is mountedon a movable support connected to the fast support of the first set by asingle-action hydraulic jack, the working chamber of said jack beingconnected to at least one oleopneumatic accumulator adjusted to a givenpressure, in order that, at the time of the loading operation, thetension of the cable causes first the shifting of the movable support ofthe second set of sheaves towards the first set of sheaves, and then,only when said support has reached a position wherein the pressuregenerated in the accumulators produces at the level of the cables thetension required for lifting the load, the elevation of said load.

The invention will now be described in further detail, with reference tothe appended diagrammatic drawing, which illustrates, by way ofnonlimiting example, the application of the device of the invention to atower crane having a horizontal crab jib. In the drawing:

FIG. 1 is a side elevational view of said tower crane,

FIG. 2 is a side elevational view, on an enlarged scale, of theload-carrying jib, and shows the various sheaves returning the hoistcable,

FIG. 3 is a diagram illustrating the path of the hoist cable,

FIGS. 4 and 5 are a side elevational view and a top plan view,respectively, of the device for slackening the tension of the cable, asshown on an enlarged scale.

The tower crane illustrated in FIG. 1 consists, in a known manner, of atower 2, at the upper part of which a tower head 3 is mounted, whichhead is integral with the load jib 4 and the counterweight jib 5. Acarriage 6, which carries the lifting hook 7, is adapted to travel onthe load jib 4. The counterweight jib 5 is likewise provided with acarriage 8, which carries a counterweight 9, which carriage is adaptedto be shifted between two positions A and B through the agency of awinch 10 and a cable 12.

According to the invention, said crane is provided with a deviceensuring the permanent equilibration thereof, whatever the load carriedby the crane and the position of the carriage 6 on the jib 4, and,therefore, the moment of said load with respect to the verticallongitudinal axis of the crane, may be.

Said device is essentially constituted by three means:

an extensometer l3 interposed between the two parts of a tie l4connecting the jib 4 to the tower head 3,

the above-mentioned winch l0,

a device 16 for slackening the tension of the hoist cable 15, whichdevice acts at the time the load is being loaded.

On the other hand, the hoist cable 15 follows an especially long path,as shown in FIGS. 2 and 3. As a matter of fact, said cable starts from awinch l7 and passes over a sliding sheave 18, over two sheaves 19disposed at the top of the tower head 3, and over a sheave 20 disposedat the fixed end of the jib 4, and then over the sheaves 22 of a blockintegral with the lifting hook 7. From said block, the cable 15 passesover a sheave 23 disposed at the free end of the jib 4, and then, afterpassing over guiding sheaves 24, reaches the slackening device 16disposed downstream of the anchorage point for said cable.

Said slackening device 16 comprises an eight-strand block having itsfirst set of sheaves 26 keyed to a fast support 27, and its second setof sheaves 28 mounted on a carriage 29 adapted to travel lengthwise overrails 30 integral with a frame 32. Said carriage 29 is connected to thefast support 27 by a single action hydraulic jack 33 provided with acompensation tank. The chamber of said jack is connected to twooleopneumatic accumulators 34 located on the frame 32, while a flowregulator (not shown) is interposed therebetween.

As a result of said arrangement, when the lifting hook 7 is hooked on toa load, and the winch 17 for the cable 15 is operated in a direction tohoist said load, the tension in said cable 15 causes first the secondset of sheaves 28 of the eightstrand block to be shifted towards thefirst set of sheaves 26 under the control of the jack 33. As a matter offact, during said shifting, the rod of the jack withdraws within thejack body, while the piston of the jack drives the oil in the workingchamber of the latter towards the accumulators 34. As a function of boththe resisting stress caused by the pressure adjustment in saidaccumulators and the force applied to the cable, the two sets of sheaves26, 28 are brought nearer to each other to a corresponding extent,whereby the distance between their centers is increased. As theaccumulators store oil from the jack, the pressure increases, and, as aconsequence, so does the stress on the cable until said stress becomesequal to the stress created by the weight of the load, whereupon themotion of the slackening device stops, while the load is lifted.

During the event of reduction of the distance between the centers of thesets of sheaves, the cable 15 passing over the block unwinds itself thusby a certain amount which, at the beginning of such motion, iscompensated by the other end of the cable winding itself on the winch17.

As a result, the load is not lifted during said event. The increase inthe duration of the loading operation is turned to account forcontrolling the shifting of the counterweight 9.

As a matter of fact, although the load is not lifted yet, the stresssaid load exerts on the jib 4 is sensed by the extensometer 13, togetherwith the position of said stress with respect to the verticallongitudinal axis of the tower 2. The extehsometer 13 determines thusthe moment resulting from said load and from the position thereof, andsends the information to a mechanical, electrical, or the like relaywhich actuates the speed reducing motor unit 10. The latter shifts thenthe counterweight 9 by the amount required for ensuring an absoluteequilibration of the crane.

As appears from the foregoing, the increase in the duration of theloading operation is of great importance, because it allows not only toshift the counterweight 9, but also to carry out said shifting operationslowly and progressively, so that there are no risks of disorderlymovements like those which would occur, due to the inherent elasticityof the various component parts of the crane, in the event of the loadbeing lifted quickly and immediately.

Thus, owing to the slackening device 16, at the time the load beginsbeing lifted the counterweight 9 is in its optimum position, and remainstherein as long as the load is not laid down.

Obviously, when the load is laid down, the reverse process takes place.The winch l7 unwinds the cable 15, while the slackening device 16 windsthe other end of the cable under the action of the accumulators 34 whichsupply the pressure required for pushing back the rod of the jack 33 andmoving away the second set of sheaves 28 from the first set of sheaves26, that is, for increasing the distance between the centers of said twosets. The laying down time increased in this way is then long enough forenabling the counterweight 9 to be shifted slowly and progressivelytowards its initial equilibrating position corresponding to the ownmoment of the tower crane.

In certain applications, and although the pressure adjustment of theaccumulators 34 enables the shifting of the carriage 29 of the device 16to be limited as a function of the load, electric or mechanical safetydevices 35, 36, are provided for stopping said shifting when the load iseither insufficient for deserving a shifting of the counterweight, ordefinite ly too high for the capacity of the slackening device.

In a modified embodiment of the present invention, the increase in theduration of the loading operation may be obtained by increasing theflexibility of the crane.

Obviously, the invention is not limited to the sole embodiment of saiddevice just described hereinabove, but covers all modifications thereofwithin the scope of the appended claims, as well as applications tocranes of other types, such as tower cranes with hinged jibs, towercranes with hinged jibs and crabs, tower cranes with retractable jibs,and tower cranes with telescopic jibs.

What is claimed is:

l. A device for obtaining permanent equilibration in tower cranes of thetype including a load jib, a carriage fitted with a load-lifting hook, awinding hoist cable connected at its free end to said carriage forimparting movement to the latter over said jib, a second jib, a carriagesupporting a counterweight adapted to be shifted over said second jib,comprising the combination of means for measuring the moment resultingfrom the weight of the load on the one hand, and from the position ofthe load carriage on the load jib on the other hand, means for shiftingthe counterweight-supporting carriage on said second jib, and means forincreasing the duration of the loading operation to enable saidlast-mentioned carriage to move slowly and progressively only inresponse to variations in the load moment while eliminating suddenmotions thereof, the increase in the duration of the loading operationbeing in response to an increase in the length of hoist cable unwoundfrom its winding end, and a device for temporarily slackening thetension in said hoist cable.

2. A device as claimed in claim 1, wherein the device slackening thetension of the cable temporarily is hydraulic, and comprises amultistrand block interposed between the fixed point of the hoist cableand the lifting hook, the first set of sheaves of said block being fast,while the second set of sheaves is mounted on a movable supportconnected to the fast support of the first set of sheaves by a singleaction hydraulic jack, the working chamber of said jack being connectedto at least one oleopneumatic accumulator adjusted to a given pressure,in order that, at the time of the loading operation, the tension of thecable causes first the shifting of the movable support of said secondset of sheaves towards said first set of sheaves, and then, only whensaid support has completed its travel, the ele at i on pf the load.

1. A device for obtaining permanent equilibration in tower cranes of thetype including a load jib, a carriage fitted with a load-lifting hook, awinding hoist cable connected at its free end to said carriage forimparting movement to the latter over said jib, a second jib, a carriagesupporting a counterweight adapted to be shifted over said second jib,comprising the combination of means for measuring the moment resultingfrom the weight of the load on the one hand, and from the position ofthe load carriage on the load jib on the other hand, means for shiftingthe counterweight-supporting carriage on said second jib, and means forincreasing the duration of the loading operation to enable saidlast-mentioned carriage to move slowly and progressively only inresponse to variations in the load moment while eliminating suddenmotions thereof, the increase in the duration of the loading operationbeing in response to an increase in the length of hoist cable unwoundfrom its winding end, and a device for temporarily slackening thetension in said hoist cable.
 2. A device as claimed in claim 1, whereinthe device slackening the tension of the cable temporarily is hydraulic,and comprises a multistrand block interposed between the fixed point ofthe hoist cable and the lifting hook, the first set of sheaves of saidblock being fast, while the second set of sheaves is mounted on amovable support connected to the fast support of the first set ofsheaves by a single action hydraulic jack, the working chamber of saidjack being connected to at least one oleopneumatic accumulator adjustedto a given pressure, in order that, at the time of the loadingoperation, the tension of the cable causes first the shifting of themovable support of said second set of sheaves towards said first set ofsheaves, and then, only when said support has completed its travel, theelevation of the load.